Vertebral fusion is a common spinal surgical procedure, performed in order to overcome problems related to impaired mutual interaction between neighboring vertebrae. The technique involves the disablement of the relative mechanical motion between adjacent vertebrae, the stimulating of growth of inter-vertebra hard tissue, causing the two vertebrae to fuse and thus preventing compression during body movements and stabilizing the spinal column. Spinal fusion is a very common procedure, with an estimated over 400,000 procedures performed annually in the USA alone.
A common method for fusing two vertebrae is by means of connecting rods, one on either side of the spine, each rod connecting two pedicle screws inserted into the pedicles of the two adjacent vertebrae. The rods are generally inserted into slots in the heads of the pre-inserted pedicle screws. Thus, in order to fuse one level, by which is meant two adjacent vertebrae, two rods, one on either side of the spine, are generally affixed to the vertebrae by means of screws inserted into the four pedicles involved, and thus connect the vertebrae rigidly, as shown in FIG. 1. There also exists a technique, known as the Dynesys® Dynamic Stabilization System, supplied by Zimmer Inc., of Warsaw, Ind., by which slight relative motion between neighboring vertebrae is maintained by the use of slightly flexible polymer connecting tubes attached to pedicle screws by means of a polyethylene cord.
The connecting rods are aligned along the spine axis, so that in a conventional open surgical procedure, an incision of about the same length of the rods has to be made in order to insert the rods and to connect them between the two vertebrae. In an adult, this generally involves an incision of at least 50 mm, generally made at the middle line, and lateral access to insert the rods is obtained by stretching of the soft tissue.
Recently, minimally invasive procedures have been developed in order to minimize the extent of the cut and still to enable insertion of the rods. For example, the CD HORIZON© SEXTANT™ Percutaneous Rod Insertion Set, supplied by Medtronic Inc., of Minneapolis, Minn., enables the insertion of a curved rod along an arc that becomes tangential to the direction of the spine in the region of the pedicle screws. The advantage is that only three small incisions on each side of the spine are required in the skin itself, two for the two legs of the SEXTANT™ and one for the entry point of the rod of the SEXTANT™ itself, but a long cut is still necessary under the skin in order to insert the rod. The length of this subcutaneous cut is necessary because of the limitation that the rod has to have a shallow arc between the pedicles, and such a large diameter arc requires a comparatively long curved pathway within the tissues of the back for its insertion.
Another procedure is that provided by Spinal Concepts Inc. of Austin, Tex., called PathFinder™ Minimally Invasive Surgery Pedicle Screws, in which an incision in the skin is made, into which a slotted guide is fitted. The rigid connecting rod is inserted down this guide and tilting during insertion to be parallel to the spine. The lower end is worked towards the further pedicle screw, spreading the soft tissue in front of its path. Spreading of tissue is assumed to be less invasive than cutting to the same extent.
However, although these prior art systems are less invasive than the classical open incision methods traditionally used, they still involve tissue and muscle trauma beyond that of simple incisions of limited size. There therefore exists a need for an even less invasive method for performing spinal fusion procedures than those currently available.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.